i965/fs: Add a couple more global special regs to special[]

Nothing bad came of this because they weren't used after visitor running,
but leaving them in a bad state seems like a recipe for pain later.

Suggested-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -1736,6 +1736,8 @@ fs_visitor::compact_virtual_grfs()
       { outputs, ARRAY_SIZE(outputs) },
       { delta_x, ARRAY_SIZE(delta_x) },
       { delta_y, ARRAY_SIZE(delta_y) },
+      { &sample_mask, 1 },
+      { &shader_start_time, 1 },
    };
 
    /* Treat all special values as used, to be conservative */